The investigation in animals of behavior thought to be characteristic of humans alone can serve to support the an evolutionary theory of similarity of behavior among species. More important perhaps, animal models of human behavior often encourage the identification of simpler accounts of human behavior than typically found in the psychological literature. For example, research with animals has forced the reexamination of the hypothesis that the ability demonstrate stimulus equivalence requires verbal ability. It has also led to accounts of the phenomenon of transitive inference that are simpler than those proposed by many developmental psychologists (e.g. propositional logic or the linear representation of stimuli). In the present proposal we will investigate in animals a social psychological phenomenon that, when found in humans has been called "cognitive dissonance." More specifically, "worth ethic" or "justification of effort," which can be described as the attribution of greater value to rewards that follow greater effort, has been thought to depend on the human need to resolve the dissonance presumably engendered by the exertion of greater effort to obtain comparable rewards. We have found that pigeons will demonstrate similar behavior (stimuli that follow a high effort response are preferred over those that follow a low effort response). We interpret that behavior in terms of a form of contrast, which in this case can be described as the relatively greater improvement in hedonic state experienced between the end of the high effort response and the signal for reward. We propose to (1) examine the functional relation between effort and reward value, (2) determine the source of the contrast, (3) examine the generality of the effect, (4) distinguish experimentally between effort and delay to reinforcement, (5) confirm that it is the value of the reward that is enhanced by prior effort, and (6) determine why it is that even negative discriminative stimuli show this effect. The proposed research should have important implications for theories of cognitive dissonance as well as other psychological theories of human social behavior.